WE ALL THINK OF A RADIO AS A PHYSICAL DEVICE. It used to be a big wooden
box with tubes in it, but the radio morphed into a small plastic box with
transistors and today it is — in most cases — a single integrated circuit.

But now, many of the functions
previously performed by electronic
components and circuits are being
carried out by special programs
running on a fast digital processor or,
in some cases, a fast programmable
logic device. While the radio is still
a form of hardware, that hardware is
programmable so the radio will do
what you want. This is not just some
fantasy. It is just as real as the cell
phone in your pocket. Here’s an update
on this hot wireless technology.

SDR REFRESHER

In case you forgot, or actually
in case you never knew, a software-defined radio (SDR) is considered to
be any radio receiver or transmitter
that uses software to define some of
the basic physical functions usually
carried out by traditional circuitry.
(Think filtering and demodulation.)
And since we are saying that software
defines the various characteristics of
the radio, that implies the use of
some form of microcomputer that
executes the software. In some cases,
that micro is a special digital signal
processor (DSP) or at least an
embedded controller fast enough to
do the functions in real time. It could

16 October 2008

also be a regular PC or laptop.

Many new SDRs incorporate a
programmable logic device like a
field programmable gate array (FPGA).
The bottom line is the radio essentially
just becomes another electronic
product based on a micro. What
product does NOT have an embedded
controller or other processor in it?
For discussion purposes in this article,
I am only going to cover receivers.
Transmitters are a special case.

So, can I just connect an antenna
directly to a microcomputer and make
a receiver with the proper software?
The answer is, almost. In fact, the
simplest SDR receiver is almost just that.
Since radio signals are very low level,
they almost always must be amplified
before you do any processing on
them. So, a simple SDR would start
with an RF amplifier after the antenna.

Don’t forget that radio signals are
analog signals. Microcomputers are
digital and process binary information.
So, after the RF amplifier you have to
use an analog-to-digital converter
(ADC) to translate the analog RF signals into a stream of binary numbers
that the micro can process. With the
digitized RF in memory, the processor
can go to work performing all the
remaining functions of a receiver.

The two basic receiver functions
are frequency selection (or what we
also call tuning) and demodulation.
Both of these functions can be done
in software. Frequency selection
involves filtering, typically using some
sort of band pass filter to distinguish
one signal from another. The filter
selects only the one we want and
diminishes the others.

Once the signal is selected, we
put it through a demodulation
process to recover the originally
transmitted information. If that
information was voice, then after
the demodulation process we have
a file of binary data representing that
voice. Now all we do is send that
data to a digital-to-analog converter
(DAC) that gives us the voice in
natural analog form. Usually, the
DAC output must be amplified by an
external single chip audio amplifier
capable of driving a speaker or a
headset. The big issue in all of this is
how fast we can perform analog-to-digital conversion. To accurately
represent the analog data, we must
sample it at a rate at least twice the
highest frequency content. If we want
to receive signal up to say 30 MHz,
then our ADC needs to run at least
at 60 megasamples per second